1. Field of the Invention
The present invention relates to an air-fuel ratio control system and method for an internal combustion engine, and an engine control unit, which control the amount of fuel to be supplied to a plurality of cylinders, on a cylinder-by-cylinder basis, to thereby control the air-fuel ratio of a mixture supplied to each of the cylinders.
2. Description of the Related Art
In general, in an internal combustion engine, if the air-fuel ratio of a mixture supplied to a plurality of cylinders varies between the cylinders due to malfunction of an injector, an EGR system, or an evaporative fuel progressing system, the emission reduction rate of a three-way catalyst is degraded, which increases harmful substances in exhaust gases emitted into the air. To eliminate the inconvenience, there has conventionally been proposed an air-fuel ratio control system e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-213284, which controls the air-fuel ratios of mixtures supplied to the cylinders such that they become equal to each other. This air-fuel ratio control system is comprised of an air-fuel ratio sensor disposed in an exhaust pipe to detect the concentration of oxygen in exhaust gases and output a signal indicative of the sensed oxygen concentration, first and second bandpass filters to which the output from the air-fuel ratio sensor is input, a control unit connected to the first and second bandpass filters, and a plurality of injectors connected to the control unit to supply fuel to the cylinders.
The first and second bandpass filters filter the output from the air-fuel ratio sensor such that components thereof in predetermined frequency bands different from each other are allowed to pass through the filters. The control unit calculates the oxygen concentration of exhaust gases emitted from each cylinder and a target value of the oxygen concentration of the exhaust gases, on a cylinder-by-cylinder basis, based on the filtered values of the output from the air-fuel ratio sensor. Then, the control unit determines the difference between the calculated oxygen concentration of the exhaust gases and the calculated target value of the oxygen concentration, on a cylinder-by-cylinder deviation, and controls the fuel injection amount of the injector of each cylinder based on the difference, to thereby control the oxygen concentrations of exhaust gases from the respective cylinders, i.e. the air-fuel ratios of mixtures supplied to the respective cylinders (hereinafter referred to as “the air-fuel ratios associated with the respective cylinders” or the like), such that they become equal to each other. The amount of fuel injected from each injector is thus controlled based on the values of the output from the air-fuel ratio sensor subjected to filtering by the first and second bandpass filters with a view to enhancing the robustness of the air-fuel ratio control by eliminating noise components generated due to the pressure of exhaust gases and the manufacturing tolerance or wear of intake valves from the output from the air-fuel ratio sensor by the filtering operations of the filters.
However, in the conventional control system described above, the amount of fuel injected from each injector is controlled based on the difference between the oxygen concentration of exhaust gases from the corresponding cylinder and a predetermined target value set when the cylinder-by-cylinder oxygen concentration is determined. Therefore, when the difference is very large, it takes long time for the oxygen concentrations of exhaust gases from all the cylinders to converge to the target value. As a result, it takes a longer time period to eliminate variation in air-fuel ratio between the cylinders, resulting in an increase in the amount of harmful substances emitted from the engine during the time period.